CE Harris wrote an article in American Rifleman, back in the 80's or 90's, about the Mosin Nagant. He had a custom barrel installed on one, in 7.62X 54R.
I would be curious to know how the 45/70 feeds and extracts. But if you look at a Mosin, the feed ramps, contours, they are all designed around the 7.62 X 54 R cartridge, and that is the safe bet for function.
And there is nothing "wrong" about the 7.62 X54R except for the rims. I have had rim lock with some ammunition, the rim on top locked with the cartridge below, but that is about it. The round is as powerful as a 308 and is capable of excellent accuracy.
If I'm not shooting, I'm reloading.